The article points to evidence suggesting it will be a new stationary battery storage system. The company has mentioned it was working on such a product in the past, and in February company officials said an announcement would be coming in a few months.

Then, today, Musk followed with two more tweets seeming to hint at battery storage:

SolarCity just hit a new daily energy record of 5GWh two weeks after reaching 4GWh

The April 30th announcement should give you plenty of time to bone up on your knowledge of why more battery storage for solar would be a big deal. Check out Inside Energy’s Solar Challenge series for a down-to-earth explanation.